Duplicate Tab allows you to duplicate a tab to a new tab or to a new window, and it allows you to merge different windows in one window. All three actions can be invoked through the tab's context menu or through shortcut-key's:

Accel+Shift+U to duplicate a tab to a new tab

Accel+Shift+N to duplicate a tab in a new window (Alt+Shift+N on Mozilla Suite)

Accel+Shift+M to merge windows

You can also duplicate a tab by using the toolbar buttons.

For merging you can also select tabs by ctrl-clicking on them. When you merge windows only the selected tabs will be merged with the previous focussed window.

You can also open a link in a duplicated tab, so the history of the original tab is available in the new tab.
And you can set to open middle clicked items from the drop down menu of the back and forward toolbar buttons in a duplicate tab.

The shortcut-keys can be changed with the keyconfig extension.
And the selection of tabs for merging (ctrl-click) can be changed with Tab Clicking Options.

Please visit twanno.mozdev.org for more information. Also about the options for Merging Windows, which are a bit hard to explain.

There are options added to select where you want the context menu items to appear. You can set to have them available in the browser page's context menu and/or in the tab's context menu. There is also an option to select whether you want the context menu items to appear in a Duplicate Tab submenu or not. When you select to not have a submenu you can also choose which items you want to have visible ('Duplicate Tab', 'Duplicate Tab To New Window', 'Merge Windows').

I will try to add that option in the next release. Do you want to have an extra entry in the context menu, or do you want a pref to change the target of the duplication? In other words: do you want to be able to use both functions everytime, or do you want to use only one of those functions standard?

I never was able to install extensions directly from this forum, instead I had to download them, so I don't think you were doing something wrong. The extension should work with your build (or newer builds).